1. Field of the Invention
The present invention relates to a fuel controlling system for an internal combustion engine in which the quantity of intake air in the internal combustion engine is detected by an air flow sensor and the quantity of fuel to be fed to the internal combustion engine is controlled on the basis of the detected output.
2. Description of the Prior Art
According to a conventional method of controlling the quantity of fuel to be fed to an internal combustion engine, an air flow sensor (hereinafter referred to simply as an "AFS") is disposed upstream of a throttle valve and the quantity of intake air per intake is determined on the basis of information obtained by the AFS and the engine speed to control the quantity of fuel to be fed.
In the case where an AFS is disposed upstream of the throttle valve in the air intake passage to detect the quantity of intake air for an internal combustion engine, when the throttle valve opens suddenly, the quantity of air charged into the intake passage between the throttle valve and the engine is also measured, so the total quantity of air measured will be larger than that actually introduced into the internal combustion engine, resulting in that the fuel quantity control based on such measured quantity would cause an overrich condition. According to a conventional proposal for avoiding this inconvenience, if the output of the AFS, i.e., a detected intake air quantity at a predetermined crank angle, is an AN.sub.(t), the quantity of air introduced into the internal combustion engine at n-1.sup.th time and that at n.sup.th time both of the predetermined angle are AN.sub.(n-1) and AN.sub.(n), respectively, and the filter constant is K, AN.sub.(n) is calculated according to the following equation and fuel control is made using the calculated AN.sub.(n) : EQU AN.sub.(n) =K.sub.1 .times.AN.sub.(n-1) +K.sub.2 .times.AN.sub.(t)
This is for smoothing the intake air quantity at every predetermined crank angle to effect an appropriate fuel control.
According to the above conventional fuel control system, however, a relatively large amount of time is required for the calculation of the air quantity, so in the event of variation in the number of revolutions caused by disturbance such as a change of the road surface, for example in a very low speed condition of a vehicle, the air fuel ratio cannot follow such variation and changes in a direction to enlarge the change in the number of revolutions and thus the revolution generating condition cannot be controlled. For more detailed explanation, reference is here made to FIGS. 1 and 2. In the characteristic diagram of FIG. 1, (a) represents the number of revolutions, Ne, (b) represents the pressure of an intake pipe, (c) represents the width of a driving pulse for an injector, and (d) represents the air fuel ratio. Usually, when the number of revolutions, Ne, changes, the pressure of the intake pipe changes somewhat later than that under the influence of the intake pipe volume. The quantity of air introduced into the internal combustion engine also lags behind the number of revolutions, Ne, in proportion to the intake pipe pressure. When correction is made according to the foregoing equation, the air quantity lags behind the intake pipe pressure and a pulse width signal for the injector also lags as shown in (e). At this time, when the number of revolutions, Ne, is high, the air fuel ratio changes to the rich side, while when the number of revolutions, Ne, is low, the air fuel ratio changes to the lean side, as shown in (g). Consequently, the characteristics of the internal combustion engine shown in FIG. 2 allow the variation in the number of revolutions to be promoted, resulting in that the driving condition becomes very unstable.